1. Field of the Invention
The present invention relates to a method and an apparatus for extruding a powdered material to form a pellet.
2. Description of the Prior Art
In the field of powder metallurgy, a powdered material is pressed into a pellet with a desired shape before sintering, and a compression molding method is popularly used for compacting the powdered material. In this method, a powdered material is poured into a molding cavity and compressed by a punch applied to the cavity. However, if an elongated pellet is desired and the molding cavity has thus a large longitudinal depth in comparison with the area of the surface pressed by the punch, it is difficult to uniformly press the whole of the powdered material in the molding cavity. As a result, the use of this method is limited only for manufacturing a pellet which has a small length relative to the area of the pressed surface. Therefore, when a longitudinal pellet is the goods, an extrusion method has been conventionally used.
In the conventional extrusion method, the powdered material is ordinarily mixed with a binder such as a wax and the like for imparting moldability to the powdered material, and converted into a paste. Then, the obtained paste is filled into a container of an extruder in which the extrusion operation is performed in a manner similar to that of an injection molding machine, and the paste is extruded from the container through a cavity to form an extrudate, which is sintered thereafter to obtain a sintered product. In most cases, it is, however, required to subject the extrudate to a aftertreatment to remove the binder from the extrudate.
Moreover, in a case of employing a hot extrusion technique, the kind of raw powdered material and the operating temperature must be carefully selected so that oxidation of the powdered material can be avoided. However, if such a temperature selection is difficult and the use of a material which is expected to be oxidized at a high temperature is unavoidable, an encapsulation method must be employed. In the encapsulation method, the raw material is enclosed in a sheath under vacuum, and is then subjected to hot extrusion. The above treatment prevents the raw material from being oxidized. However, except for some special cases, the extrudate should be further treated to remove the sheath by chemical dissolution, etc.
As described above, for implementing the conventional extrusion method, a variety of additional and complicated steps, i.e. the paste-forming step, the wax-removing step, etc. are necessary before or after the essential extrusion process. Accordingly, the conventional extrusion method, particularly a hot extrusion method, has an adverse effect on the manufacturing cost. Therefore, at present, the technical field in which the extrusion method can be employed is limited to only special cases in which the manufactured pellet has a high value added for economic reasons and in which the pellet manufactured by using the extrusion method can be imparted a particular property that cannot be obtained by the ordinary powder metallurgical method. Consequently, the extrusion method has not been generally utilized for manufacturing ordinary machine parts, in spite of prominent characteristics latent in that method.